Process of treating amalgams of metals which are difficultly soluble in mercury



United States Patent C PROCESS OF TREATING AMALGAMS F METALS gggfl AREDIFFICULTLY SOLUBLE IN MER- No Drawing. Application July 25, 1957 SerialNo. 674,044

Claims priority, application Austria July 27, 1956 7 Claims. (CI. 7563)This invention relates to a process of treating amalgams of metals whichare diflicultly soluble in mercury.

In this specification the term metal has the meaning metals other thanmercury and the phrase diflicultly soluble includes the meaning ofinsoluble.

It is a main object of the invention to reduce the amount of energyrequired for the recovery of metals which are difficultly soluble inmercury from dilute amalgains.

It is another object of the invention to provide a process ofconditioning a dilute amalgam of a metal which is dificultly soluble inmercury for separation of the mercury therefrom in the liquid phase to aconcentration of said metal which was previously achievable only bydistilling the mercury.

It is another object of the invention to provide an improved process ofseparating metals which are difiicultly soluble in mercury from theiramalgams by a superficial oiddation of said metals.

Other objects and advantages of the invention will become apparent asthe specification proceeds.

In amalgam metallurgy, metallic mercury is used as an auxiliarysubstance in the recovery of metals. The amalgam-metallurgical recoveryof such metals always proceeds in two stages. In the first stage thecorresponding metal amalgams are formed, e.g., by direct electrolysis,by phase exchange reactions with less noble amalgams and the aqueousmetal salt solutions, by the reduction of metal compounds with the aidof reducing amalgams, and by other methods. In the second stage themetal to be recovered must be separated from the mercury with which itis amalgamated. Metals which are difiiculty soluble in mercury maypreferably be recovered from their amalgams by distilling the readilyvolatile mercury. Although the latent heat of vaporization of mercury issmall, a certain amount of energy can still be saved during thelow-temperature distillation if amalgams of the highest possibleconcentration are subjected to distillation. The known amalgamatingreactions generally git. nalgams having concentrations of no more thanbetween t and two percent by weight. Amalgams containing t: by weight ofmetals which are difiicultly soluble in mercury have a highly viscous topasty consistency. However, they may be enriched to a content of between2 percent and, at most, 4-5 percent by weight of metal in mercury bysimple known liquid-solid separation processes such as filtering,centrifuging, ferromagnetic separation. All these processes give amercury which is al most free of metal and a stifl amalgam paste ofbutterlike consistency. No processes have been known before which enablea more effective separation with such simple means. Therefore, thematerial subjected to the previous known low-temperature distillationprocesses consisted always of amalgams of relatively low concentration,containing between 0.5% and 5% to a maximum of by weight of metal. Byway of example it may be stated that a 1% iron amalgam obtained bydirect electrolysis could not be enriched by filtration to an ironcontent of more than 8-9 percent iron even under very high pressures (70kg./sq. cm.). This 8-9% iron amalgam has already a friable consistencyand is no longer capable of flowing.

It is also known that amalgams having a small content of metals whichare very difiicultly soluble in mercury can be aged to some degree byprolonged storage. Enriched amalgams containing up to as much as 5-10%metal can then be obtained by filtering aged amalgams. More highlyenriched amalgams obtained without distillation of the mercury have notbeen disclosed so far. It has already been disclosed to age amalgams atan elevated temperature in order to facilitate the floating of the metalparticles that are insoluble in mercury. This measure, however, does notlead to a further enriching of the metal in the amalgam.

Surprisingly it has been found that the heating preferably above 450 C.,under a pressure which is equal to or higher than the vapor pressure ofthe mercury corresponding to this temperature, whereby an evaporation ofthe mercury is prevented, results in such a coarsening of the grains ofthe metal which is difficultly soluble in the mercury that highlyconcentrated amalgams result from the subsequent separation of the majorportion of the liquid mercury by known methods, e.g. by filtration.These metal amalgams contain 15-50% metal. If the starting amalgam washeated under pressure above 450 C. they will contain in almost all casesmore than 25% metal. The residual mercury may be removed in a mannerknown per se by distillation. This process step alone enables a savingof 50-90% of the energy used for lowtemperature distillation. Thesuperatmospheric pressure required during the heat treatment of theamalgam may be produced by heating the amalgam above the boiling pointof mercury in a pressure-tight chamber so that the heated amalgam issubjected to the vapor pressure of the mercury. If the heat treatment iscarried out in a continuous operation the incoming and outgoing amalgamsmay be subjected to counterflow heat exchange with each other forpreheating the incoming amalgam and cooling the outgoing amalgam. Inthis case the heat-treating chamber may be sealed with liquid amalgam atthe feed and discharge ends. The processaccording to the invention maybe illustrated by the following examples:

Example 1 Example 2 Nickel amalgam containing 4% nickel was heated underpressure for five minutes at 550 C. Filtration after cooling gave apasty filtration residue containing 31% nickel.

Example3 The same nickel amalgam as in Example 2 was treated for onehour at 440 C. Filtration at room temperature gave a residue containing20% nickel and having almost the same consistency as that obtained inExample 2.

Example 4 Copper amalgam containing 3.5% copper was treated for 15minutes at 530 C. The filtration residue" contained 30% copper. I

of these iron particles by mercury.

cold.-' The filtration residue contained'only ll manganese l a iExample. 6

' The same manganese amalgam as inExample was heated at 560 C. andfiltered at 280 C., at a temperatore below the boiling point of mercury.The filtration residue contained 35 percent manganese.

The application of higher temperatures during the heat treatment mayincrease the concentration of metal in the l ltiation' residue to asmuch as 50 percent. In practice,

however, 600 C. will be greatly exceeded because the vapor pressure ofmercury increases with temperature to such a degree thatits controlunder commercial conditions The present inventionincludes an additionalimportant feature which consists in enriching the metal content ofamalgams by oxidation. The effect described is most easily observed withchromium amalgam which will separate into a gray chromium powderand-chromiumfree mercury after a short period of standing on the air.lrcnamalgam must he allowed to stand in the air for several days or evenweelzs toeifect this separation. The iron which has been separated inthis way from the mercury with which it had been amalgamated has alreadybeen investigated. It was found to consist of metallic iron having anradiologically'amorphous xide layer or oxygen covering layer,which'eliminates the wettability Previous attempts to commerciallyutilize this effect for the removal of mercury from metal amalgams haveremained without any practical success because this separation is notcompleted even within a very long time, particularly with those amalgamswhich are of commercial interest (amalgams of iron, nickel, manganese,copper and others).

Surprisingly it has been found that the amalgams which have beenenriched according to the invention by filtering dilute amalgams after aheat treatment, are under certain conditions particularly suitable foroxidative separation. It has been found that this oxidative separationofamalgaz s' which had been treated at elevated temperature undcrpressure will occur particularly quickly and completely if the mercurycontains in solution a small amount of a metal which has a higherailinity to oxygen than the metal to be recovered from the amalgam. Forthe effect of the oxidative separation it is of minor importance whetherthis metal of higher to oxygen is added to the mercury beforethepressure treatment or after the pressure ..treatment before theoxidation. Aluminum, e.g., has proved particularly suitable as anaddition to pressure-enriched amalgams to promote their oxidativeseparation. It has been found that an addition of to 10-. percentbyweigbt of aluminum, related to the mercury, has a particularlydesirable effect. An addition of manganese in an order of 10- to 10-percent by weight is also suitable. Manganese is preferably incorporatedin the amalgam by a direct electrolysis of a manganese-conteining ironsalt solution. To incorporate aluminum, it may bedissolved in mercury atelevated temperatures or may be added to the mercury before the;

heat treatment under pressure. It has been found that the separationdepends to a high degree on the residence time of'the amalgam in thepressurized heat-treating zone after an addition of aluminum. Thefollowing example illn strates the enriching effect on iron amalgams towhich tracesof aluminum had been added before the heat treatment:

I Example 7 iron amalgamhaving an iron content of 2.5% was; treated forten minutes at 550 C. under pressure and;

4 Y was then filtered. A separation to as much as 60 percent by weightof metal Was possible on the filter.

7 Example 8 When'the same iron amalgam as in Example 7 was heat-treatedfor two hours, it was possible to enrich it by filtration only to asmuch as 60 percent by weight of metal.

Example 9 by filtration only to as much as 40 percent by weight ofmetal. 7

Example 10 When the same. amalgam as in Examples 7 to 9 was heat-treatedfor five hours, it was possible to enrich it by i has been foundsatisfactory, therefore, to add the oxygen transmitting agent to themercury after the pressure treatment. To this end, aluminum may bedissolved in mercury at 500 C. and the resulting aluminum analgam may beadmixed in the cold to the pressure-treated iron amalgam.

The metal powder obtained by oxidative separation is not actuallyoxidized but has only a oxygen covering on the several metal grains. Itis suitable as a starting product for sintered bodies.

What is claimed is: a l

1. A process of recovering a metal which is difliculty soluble inmercury from an amalgam containing said metal in the form of distinctsolid grains by adding to said amalgam a metal which has a higheraifinity to oxygen than said dificulty soluble metal and which is atleast partly soluble in mercury, then separating mercury in the liquidphase from said amalgam to form a residual amalgam enriched with saiddifl'lcultly soluble metal, allowing said diific'ultly soluble metal insaid residual v amalgam to oxidizeto obtain an-oxide-covered amalgampowder and, mercury which is substantially free of said difiicultlysoluble metal, and separating said amalgam powder from said mercury. 2.A process of recovering a metal which is difiicultly soluble in mercuryfrom an amalgam by adding to said amalgam a metal which has a higherafiinity to oxygen than said difficultly soluble metal and which is atleast partly soluble in mercury, then heating said amalgam.

7 above 360 C. at a pressure sufiicient to keepthe mercury liquid andthen separating mercury in the liquid 5'"; from said amalgam enrichedwith said difilcultly floiuble metal, allowing said diflicultly solublemetal in and residual amalgam to oxidize to obtain an oxide-coveredamalgam powder and mercury which is substantially free of saiddifiicultly soluble metal, and separating said amalgam powder from saidmercury.

. 3. In aprocessiof recovering a metal which is difiicultly soluble. inmercury from an amalgam by heating said amalgam above 360 at a pressuresufficient to keep the mercury liquid, the improved'step consisting inadding thereafter to said amalgam'a'metal'which has a higher afiinityto'oxygen than said difiicultly soluble metal and which is at leastpartly soluble in mercury, then separat ing mercury in the liquid phasefrom said amalgam en- T riched with said difl'icultly soluble metal,allowing said difiicultly soluble metal in said residual amalgam tooxidize to obtain an oxide-covered amalgam powder and mercury which issubstantially free of said difiicultly soluble metal, and separatingsaid amalgam powder from said mercury.

4. In a process of recovering a metal which is difficultly soluble inmercury from an amalgam by heating said amalgam above 360 C. at apressure sufiicient to 7 keep the mercury liquid, then separatingmercury in the liquid phase from said amalgam to form a residual amalgamenriched with said difiicultly soluble metal, the improved stepconsisting of adding thereafter to said residual amalgam a metal whichhas a higher afiinity to voxygen than said difiicultly soluble metal andwhich is metal of higher afiinity consists of aluminum and is added inan amount of 10' to 10-? percent by weight related to the mercurycontained in said amalgam.

6. A process as set forth in claim 1, in which said metal of higheraifinity consists of manganese and is added in an amount of l0- to 10*percent by weight related to the mercury contained in said amalgam.

7. A process as set forth in claim 1, in which mercury is distilled fromsaid separated amalgam powder.

References Cited in the file of this patent UNITED STATES PATENTS2,758,921 Schmidt Aug. 14, 1956 FOREIGN PATENTS 502,939 Canada May 25,1954

1. A PROCESS OF RECOVERING A METAL WHICH IS DIFFICULTY SOLUBLE INMERCURY FROM AN AMALGAM CONTAINING SAID METAL IN THE FORM OF DISTINCTSOLID GRAINS BY ADDING TO SAID AMALGAM A METAL WHICH HAS A HIGHERAFFINITY TO OXYGEN THAN SAID DIFFICULTY SOLUBLE METAL AND WHICH IS ATLEAST PARTLY SOLUBLE IN MERCURY, THEN SEPARATING MERCURY IN THE LIQUIDPHASE FROM SAID AMALGAM TO FORM RESIDUAL AMALGAM ENRICHED WITH SAIDDIFFICULTY SOLUBLE METAL ALLOWING SAID DIFFICULTY SOLUBLE METAL IN SAIDRESIDUAL AMALGAM TO OXIDIZE TO OBTAIN AN OXIDE-COVERED AMALGAM POWDERAND MERCURY WHICH IS SUBSTANTIALLY FREE OF SAID DIFFICULTY SOLUBLEMETAL, AND SEPARATING SAID AMALGAM POWDER FROM SAID MERCURY.